Human Eye And Colourful World

He is suffering from myopia and is using a double concave lens because the power of lens is -1D and focal length = so the focal length is negative indicating that it is a diverging lens.

The visible spectrum of light consists of 7 colours. These are mentioned along with the wavelength as below:

a. As the red colour has the maximum wavelength, it will deviate the least. It has less velocity, and thus deviated the least.

b. As the violet colour has the minimum wavelength, it will deviate the most.

Small wavelengths have larges velocity due to which is it deviated the most.

Pupil controls the amount of light which is entering the eye. A person pupil’s become smaller in bright light & large in dim light. As shown in the diagram below:

Reflection, diffraction and refraction are the phenomenon involved in the formation of rainbow. White light from the sun enters the top surface of the water droplet. Due to refraction, the sunlight disperses into seven colours & strikes the surface of droplet at point C. At point C, the angle formed by incident ray is greater than the critical angle. Therefore , total internal reflection occurs at point C and the dispersed spectrum of lights are reflected at the bottom surface of the droplet. At point D, the spectrum again experiences refraction and as they are travelling from denser to rarer medium, the angle between red & violet increases as compared to the angle for the first refraction. There is one more condition, that is, bright rainbow is seen only when the angle between the incoming & the outgoing rays have the maximum angle of 42 degrees.

Scattering is the phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter.

Scattering of light depends on the

i. Size of the particle which scatters the light.

When the scattering cross-section of the particles is small, less light is scattered.

ii. Wavelength of light.
Light with a higher wavelength is scattered less than light with a lesser wavelength.

Using lens formula:

For A: Object distance, u = -22.5 cm
Image distance, v = ?

v= = 45cm

Hence A’s observation is correct.

For B: Object distance, u = -25 cm
Image distance, v = ?

v= = 37.5cm

Hence B’s observation is wrong.

For C: Object distance, u = -37.5 cm
Image distance, v = ?

v= = 25cm

C’s observation is correct.

For D: Object distance, u = -45 cm
Image distance, v = ?

v= = 22.5 cm

D’s observation is also correct.

The boy is suffering from myopia. Myopia or near-sightedness or short-sightedness is a defect in which a person is unable to see far away things clearly. In this defect, a person can see objects placed at a certain distance ( far point) from the eye only but everything beyond that distance appears blurry. The maximum distance up to which a person can see things clearly that is the image is formed on the retina is called the far point.

Actually what happens in myopia is the rays coming from the object, after refraction from the lens, converges to a point before the retina as the retina is the screen where the image should be obtained. In other words, the focal length of the eye lens decreases. So the rays are focused before retina and the image is not obtained clearly leading to a blurry image. Now the main objective is to focus the light on the retina. So a double-concave lens is used in front of the eye to fulfill this purpose. A double concave lens spread the light coming from the far away placed object that makes the light rays parallel so that the rays converge at the retina and thus forming a clear image. In other words, the concave lens brings the image of the object, placed beyond the far point, in between the far point & the least distance of distinct vision. Now this image acts an object for convex lens and image is formed on the retina. The focal length, f of the lens should be equal to the negative of distance of a far point , D.

f = -D

Object distance, u = 25 cm

The distance of near point, d = 50 cm

Focal length, f = ??

Using the formula: f =

f = = = 50 cm

Power = = = 2D

As focal length is positive therefore he uses double convex lens.

Since the eye is myopic, hence, the image should be formed at 1.5 m or before that on the same side as the object.

Distance of far point, v = -1.5m = -150 cm

Object distance , u = ∞

Now, to find focal length we apply the formula:

⇒

⇒

⇒ f = -150 cm

Now,

The power is given as:

Power =

The lens used is as double concave lens.

a. Human eye contains a natural convex lens & the retina is the screen. A normal person can see things clearly beyond 25 cm which is the least distance of distinct vision.

b. Long-sighted eye means the person is suffering from hypermetropia. The least distance of distinct vision increases in hypermetropia and the image is formed beyond the retina. In this defect, the light rays coming from the object placed between near point & the point of least distance of distinct vision, after refraction from the eye lens converges to a point beyond the retina. This implies that the image is formed beyond the eye thus creating a blurry image. In such cases, the focal length of the eye lens increases. So to bring the image on retina a double convex lens is used. The double convex lens converges the light rays coming from the object in front of the eye lens. In other words, the convex lens creates an image of the object at the near point. Now this image acts as an object for the eye lens and thus forming an image on the retina.

When a white light passes through a prism, it refracts and disperses into seven colors. As different colour have different wavelengths, therefore, its velocity also changes inside the prism resulting deviation at different levels.

The sunrise is advanced or delayed due to atmospheric refraction of sunlight. An observer on the earth sees the sun two minutes before the sun reaches the horizon because a ray of sunlight entering the earth's atmosphere follows a curved path due to atmospheric refraction before reaching the earth. The density of atmosphere varies with altitude which affects the refractive index of the atmosphere. The light from the sun enters rarer to denser medium and hence bends towards the normal. Same is the case with delayed sunset.

The splitting of white light into seven colours on refraction is called dispersion of light.

The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. The amount of refraction depends on the speed of coloured light in glass.

The two prisms should be identical because if they are different, their refractive index would be different resulting in further dispersion and not recombination. If the prisms are identical then both have equal and opposite refractive index which would help in recombination. Prisms are inverted so that from first prism the lights will get diverged and from the second it will converge.

i. The figure represents Hypermetropia. Hypermetropia or far sightedness is an eye defect in which a person is not able to see things placed closer to the eye. The light rays coming from the object placed between near point & the point of least distance of distinct vision, after refraction from the eye lens converges to a point beyond the retina. This implies that the image is formed beyond the eye thus creating a blurry image. In such cases the focal length of the eye lens increases.

ii. Some of the causes of hypermetropia are:

•Low converging power of eye lens because of weak action of ciliary muscles.

•Abnormal shape of the cornea

iii. The defect can be corrected by using the convex lens. To bring the image on retina a double convex lens is used. The double convex lens converges the light rays coming from the object in front of the eye lens. In other words, the convex lens creates an image of the object at the near point. Now this image acts as an object for the eye lens and thus forming an image on the retina.

Eye consists of following parts:

Cornea: The front surface of the eye is covered by a transparent spherical membrane called the cornea. Light enters the eye through the cornea. It acts as a window that controls and focuses the entry of light into the eye. It also acts as a protective layer to the eye.

The space behind the cornea is filled with a liquid called aqueous humour.

Iris: The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris.

Pupil: Pupil is the small circular opening of iris. The pupil appears black because no light is reflected from it.

Eye lens: The eye lens is a convex lens made of a transparent jelly-like protein. The eye lens is held in position by ciliary muscles. The ciliary muscles help in changing the curvature and focal length of the eye lens.

Retina: The inner back surface of the eye ball is called retina. It is a semi-transparent membrane which is light sensitive. The light sensitive receptors of the retina are called rods and cones. When light falls on these receptors they send electrical signals to the brain through the optic nerve. The space between the retina and eye lens is filled with another fluid called vitreous humour

Blind spot: It is a spot at which the optic nerve enters the eye and is insensitive to light.

The light coming from an object enters the eye through cornea and pupil. Amount of light entering the eye depends on pupil and cornea. The eye lens converge these light rays to form a real, inverted and diminished image on the retina.

The light sensitive cells of the retina get activated with the incidence of light and generate electric signals.

These electric signals are sent to the brain by the optic nerves and the brain interprets the electrical signals in such a way that we see an image which is erect and of the same size as the object.

•When a person is trying to see far away objects, the ciliary muscles relaxes itself so that the lens elongates. This elongation of lens increased the focal length of the eye lens & thus we are able to see objects placed very far.

•The ciliary muscles contract itself if a person has to see nearby objects. This makes the lens more curved which causes the focal length to shorten itself so the person can see nearby objects.

•This is the reason why we are unable to see objects kept in room for a short time when we enter the room from outside. It takes a short time to accommodate the lens of eyes.

focal length, f =

i) For distant vision: focal length, f = = = 0.18 m

ii) For near vision: focal length, f = = = 0.67 m

i. The sky appears dark to the astronaut because in space there is no atmosphere which contains fine particles and air molecules and therefore there is no scattering of light. We are able to see the sky blue due to scattering of light by the fine particles and air molecules present in the atmosphere. Scattering phenomenon occurs only when the size of the atom is comparable to the wavelength of light.

ii. During sunrise or sunset, the sun is close to horizon and therefore the light rays coming from the sun have to travel larger part of the atmosphere. As the light travels through the atmosphere, all the other colours except red get scattered away by the air molecules and fine particles present in the atmosphere. Red colour has the longest wavelength and hence is less scattered and therefore reaches our eyes.

iii. Stars appear to twinkle because stars are very far away from the earth and emit their own light. As they are very far they are considered as point objects. The light coming from stars has to go through different density or layers of earth’s atmosphere and therefore is refracted multiple times. So sometimes the refracted light has more intensity therefore stars appear brighter and sometimes the refracted light has less intensity which makes the star dull. This causes the twinkling effect.

The labelled diagram of the human eye:

The various function of parts of human eye

i. Cornea: The front surface of the eye is covered by a transparent spherical membrane called the cornea. Light enters the eye through cornea. It acts as a window that controls and focuses the entry of light into the eye. It also acts as a protective layer to the eye.

ii. Iris: The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris.

iii.Crystalline (Eye) lens: The eye lens is a convex lens made of a transparent jelly-like protein. The lens, by changing shape, change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina.

iv. Ciliary muscles: The ciliary muscles are a ring smooth muscle in the eyes middle layer that accommodates the focal length of the eye lens so that the eyes can see at various distances.

•The ciliary muscles regulate the flow of aqueous humor.

•It controls the accommodation of the eye lens.

•When a person is trying to see far away objects, the ciliary muscles relaxes itself so that the lens elongates. This elongation of lens increased the focal length of the eye lens & thus we are able to see objects placed very far.

•The ciliary muscles contract itself if a person has to see nearby objects. This makes the lens more curved which causes the focal length to shorten itself so the person can see nearby objects.

•This is the reason why we are unable to see objects kept in room for a short time when we enter the room from outside. It takes a short time to accommodate the lens of eyes.

v. Retina: Retina acts as a screen of the eye. The purpose of the retina is to receive light that the lens has focused, convert the light into neural signals, and send these signals on to the brain for visual recognition.

The person is suffering from hypermetropia. Hypermetropia or far sightedness is an eye defect in which a person is not able to see things placed closer to the eye. A normal person can see things clearly beyond 25 cm (least distance of distinct vision). If an object is placed between the least distance of distinct vision & the near point (the minimum distance up to which the objects can be seen clearly) then a person suffering from hypermetropia will not be able to see such things. Some of the causes of hypermetropia are:

•Low converging power of eye lens because of weak action of ciliary muscles.

•Abnormal shape of the cornea

In this defect, the light rays coming from the object placed between near point & the point of least distance of distinct vision, after refraction from the eye lens converges to a point beyond the retina. This implies that the image is formed beyond the eye thus creating a blurry image. In such cases the focal length of the eye lens increases. So to bring the image on retina a double convex lens is used. The double convex lens converge the light rays coming from the object in front of the eye lens. In other words, the convex lens creates an image of the object at the near point . Now this image acts as an object for the eye lens and thus forming image on the retina.

The least distance of distinct vision, d = 1m = 100 cm
focal length, f = ?
Using formula: f =

f = = = 33.33 cm

Power = = = 3D

The person is suffering from hypermetropia as he is not able to see objects placed closer to the eyes. Hypermetropia or far sightedness is an eye defect in which a person is not able to see things placed closer to the eye. A normal person can see things clearly beyond 25 cm (least distance of distinct vision). If an object is placed between the least distance of distinct vision & the near point (the minimum distance up to which the objects can be seen clearly) then a person suffering from hypermetropia will not be able to see such things. Some of the causes of hypermetropia are:

•Low converging power of eye lens because of weak action of ciliary muscles.

•Abnormal shape of the cornea

In this defect, the light rays coming from the object placed between near point & the point of least distance of distinct vision, after refraction from the eye lens converges to a point beyond the retina. This implies that the image is formed beyond the eye thus creating a blurry image. In such cases the focal length of the eye lens increases. So to bring the image on retina a double convex lens is used. The double convex lens converge the light rays coming from the object in front of the eye lens. In other words, the convex lens creates an image of the object at the near point. Now this image acts as an object for the eye lens and thus forming image on the retina. The focal length of the convex lens should be:

f =

Where d is the least distance up to which the person is able to see.